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Life Cycle of an Alaskan Brown Bear

By Lauren Corona

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Roughly 300,000 brown bears (Ursus arctos) live in Alaska, which is around 98 percent of the total population in the United States. They can be found everywhere across the state, except on islands south of Frederick Sound or west of Unimak, and on Bering Sea islands. Those living on the Kodiak Archipelago are considered a separate subspecies (Ursus arctos middendorffi), as they've been isolated from other bears for roughly 12,000 years.

Making Babies

Breeding season for brown bears in Alaska starts between May and July. They're known as serial monogamous breeders, which means that females breed with one mate at a time, but may breed with several males over the course of her 10- to 30-day period of estrus. Females mark their territories with urine to signal that they're receptive to mating. Males will fight with each other for females, and may guard them for up to three weeks.

Bearing Young

Although the brown bear mates between May and July, implantation of the fertilized egg in the womb is delayed, and it doesn't reach the embryonic stage until October or November. During the female's gestation period, she will be safely hibernating in her den. She usually gives birth in her den to between one and four cubs—two or three being the norm—between January and March. Cubs weigh just 13 ounces, and are the smallest of all young mammals relative to parental size.

Juvenile Bears

Young Alaskan brown bears nurse in their den until June, when they're large enough to emerge with their mother. At this stage cubs weigh an average of 15 pounds. They're relatively independent, but still rely on their mothers for food and protection. Although they're generally fully weaned by 18 months of age, juvenile bears stay with their mother until at least the second spring of their life, but more often until they're 3 or 4 years old. At this stage their mother will drive them off so that she's able to mate again.

All Grown Up

Most brown bears will reach sexual maturity at roughly 5 years of age. However, many females won't mate successfully until slightly later in life. Although sexually mature, the young bears will continue to grow and won't get to their full adult size until they're roughly 10 to 11 years old. The life-span of brown bears varies, but the oldest recorded female in Alaska reached 39 years of age and the oldest Alaskan male reached 38.